DE3879349T2 - N- (L-ASPARTYL) AMINO ALCOHOL DERIVATIVES AND SWEETENERS CONTAINING THEM. - Google Patents

Description

The present invention relates to (2R,3R)-3-N-(L-aspartyl)amino-1- (1-methyl-2-norbornyl)-2-butanol or (2R,3R)-3-N-(L-aspartyl)amino-1-(3-methyl-2-norbornyl)-2-butanol, each of which has excellent sweetening properties and is widely useful in, for example, foods and medicines as a sweetener which is very stable in aqueous solutions, as well as a sweetener containing them.

In addition to sucrose, which is the most widely used good sweetener, various sweeteners have been used, including sugars such as fructose, isomerized sugars and glucose, sugar alcohols such as sorbitol and mannitol, natural sweeteners such as glycyrrhizin, stevioside and thaumatin, and artificial sweeteners such as saccharin sodium, sodium cyclamate, acesulfame-K (3,4-dihydro-6-methyl-1,2,3-oxathiazin-4-one 2,2-dioxide potassium salt) and aspartame.

Recently, there has been an increasing number of patients suffering from tooth decay as well as obesity, diabetes, heart disease, hypertension and kidney disease, which are mainly caused by excessive intake of calories. Thus, it has been necessary to develop low-calorie sweeteners for treating these diseases or maintaining good health. As a result of attempts to meet the above requirements, some products have been brought to the market. However, artificial sweeteners are subject to strict regulations due to their toxicity. On the other hand, natural sweeteners are not satisfactory in terms of their sweetness properties and aftertaste. In addition, they are expensive.

Recently, the use of L-aspartyl-L-phenylalanine methyl ester, hereinafter referred to as aspartame or "APM" This compound will become a major artificial sweetener.

More than 500 dipeptides including APM have been described (see Ariyoshi, Kagaku Sosetsu, No. 14, 85 to 128 (1976); Iwamura, J. Med. Chem., 24, 572 to 583 (1981); and A. Van der Heijden et al., Chemical Senses and Flavor, 4(2), 141 to 152 (1979)). However, there are few substances with a sweetening power of five hundred times or more that of sucrose. Furthermore, these dipeptides, which are esters, would be hydrolyzed in aqueous solutions or undergo an alcohol-removing reaction with an amino group of an aspartate residue to thereby form a diketopiperazine derivative, which would often eliminate or alter their sweetening properties.

Among the dipeptide esters known to date, L-aspartyl-DL-aminomalonic acid methylfenchyl diester, described by Fujino et al. (compare JP-B-52-34622, JP-A-49-30566 and Chem. Pharm. Bull., 24(9), 2112 (1976)) has the highest sweetening power, although it is inferior to APM in terms of stability at 80ºC and pH 4. (The terms "JP-A" and "JP-B" mean an "unexamined published Japanese patent application" and an "examined Japanese patent specification", respectively.) Recently, JP-A-62-30748 reported that L-aminocarboxylic acid amides of alkyl-substituted cycloalkyls or bicycloalkyl-substituted amino alcohols or amino ketones are 30 to 600 times sweeter than sucrose. Furthermore, JP-A-56-127339 described branched amides of L-aspartyl-D-amino acid, and U.S. Patent 4,423,029 described (S)-3-amino-4[(S,S)-1-(1-hydroxyethyl)alkylamino]-4-oxobutyric acid compounds. However, the sweetening power of these compounds is only 200 to 300 times that of sucrose, and thus they are not entirely satisfactory.

It is an object of the present invention to provide a sweetening compound having sweetening properties similar to those of sucrose, a high sweetening power and yet a low calorie count, and being very stable in acidic aqueous solutions, as well as a sweetener containing the same.

EP-A-0203540 describes sweeteners which are amides of alpha-aminodicarboxylic acids and have high sweetening power, low calorific value but stability at acidic pH and elevated temperatures; these include N-(alpha-L-aspartyl)-DL-2-amino-3-hydroxy-4-[(exo/endo)- 2-norbornyl]butane with the absolute configuration R,R. We have now discovered that methyl-substituted compounds having only the R,R configuration have unexpectedly high sweetness.

The sole figure of the figure shows the good stability of aqueous solutions of the compounds of the present invention at 80°C, where A shows (2R,3R)-3-N-(L-aspartyl)amino-1-[(2R,3R)-3-methyl-2-norbornyl]-2-butanol, B shows (2R,3R)-3-N-(L-aspartyl)amino-1- [(1S,2S)-1-methyl-2-norbornyl]-2-butanol and C shows 3-N-(L-aspartyl)amino-1-d-α-fenchyl-2-butanol, while APM means aspartame and AMF means L-aspartyl-DL-aminomalonic acid methylfenchyl ester.

The present inventors have previously found that an ester of L-aspartyl-D-alanine, particularly with fenchyl alcohol, generally has a high sweetening power. In order to improve both the sweetening power and the stability of this compound, attempts have been made to convert the ester portion into other functional groups such as a hydroxyl group and to synthesize and evaluate an L-aspartylfenchylamino alcohol derivative by condensing an amino alcohol derivative having a fenchyl group with L-aspartic acid. As a result, it has been found that this L-aspartylfenchylamino alcohol derivative not only has a high sweetening power and excellent sweetening properties, but is also excellent in solubility in water and stability in acidic aqueous solutions. Consequently, the inventors have applied for a patent for this compound (see JP-A-63-39846).

Subsequent investigations by the inventors have shown that the sweetness depends essentially on the absolute configuration at positions (a) and (b) in the following formula (i). For example, compounds of the following general formula:

wherein R₁ is a methyl group while R₂ is a fenchyl group,

a sweetening power as shown below (RS indicates a racemate): Sweetness

Thus, the compound having the absolute configuration (R,R) at positions (a) and (b) has the highest sweetening power, followed by that having (RS, RS). In contrast, those having (S,S), (S,R) and (R,S) each have little sweetness. The absolute configurations at positions (a) and (b) are determined by separating amino alcohol derivatives with tartaric acid to thereby give p-bromobenzoic acid amide derivatives of the following formula (ii), and then analyzing their crystal structures with X-rays:

where ... is below the plane of the paper, while is above it.

The inventors have further investigated the relationship between the structures, in particular the R₂ position in formula (i) of these compounds and their sweetness properties and obtained some findings. The sweetness properties of compounds with norbornane skeleton are evaluated by considering those that have an immediate sweet taste and less aftertaste, ie those that are similar to the sweetness properties of sucrose, as preferred. Thus, the following compounds are highly rated in this order.

Based on these findings, the inventors further investigated the relationship between the sweetness properties, particularly the sweetness power and sweetness quality, and the chemical and steric structures thereof. As a result, it was found that (2R,3R)-3-N-(L-aspartyl)amino-1-(1-methyl-2-norbornyl)-2-butanol and (2R,3R)-3-N-(L-aspartyl)amino-1-(3-methyl-2-norbornyl)-2-butanol have excellent sweetness properties, thereby achieving the present invention.

Accordingly, the present invention provides (2R,3R)-3-N-(L- aspartyl)amino-1-(1-methyl-2-norbornyl)-2-butanol and (2R,3R)-3-N- (L-aspartyl)amino-1-(3-methyl-2-norbornyl)-2-butanol and a sweetener containing them.

The following optical isomers of the 1- or 3-methyl-2-norbornyl skeleton exist:

The (2R,3R)-3-N-(L-aspartyl)amino-1-(1-methyl-2-norbornyl)-2-butanol or (2R,3R)-3-N-(L-aspartyl)amino-1-(3-methyl-2-norbornyl)-2-butanol derived from any of these 1- or 3-methyl-2-norbornyl optical isomers can be obtained by appropriate selection of the starting materials.

For example, (2R,3R)-3-N-(L-aspartyl)amino-1-[(15,25)-1-methyl-2-norbornyl]-2-butanol and (2R,3R)-3-N-(L-aspartyl)amino-1-[(25,35)-3-methyl-2-norbornyl]-2-butanol, each within the scope of the present invention, can be synthesized by the following Schemes (A) and (B), respectively, wherein Ts represents a p-toluenesulfonyl group, Cbz represents a carbobenzoxy group, and Bz represents a benzyl group. Reaction scheme (A): Lipase Peracetic acid BF₃- etherate complex KF catalyst Raney Ni (+)- and (-)- tartaric acid reduction

As shown in the above reaction scheme (A), (2R,3R)-3-N-(L-aspartyl)amino-1-[(1S,2S)-1-methyl-2-norbornyl]-2-butanol can be obtained in the following manner.

Norbornanone (I) is reacted with methylmagnesium iodide to give 2-methyl-2-norbornanol (II). The compound (II) is reacted with acetic acid and a 75% solution of sulfuric acid to give 2-acetoxy-l-methylnorbornane (III), which is then converted into 1-methyl-2-norbornanol (IV) with lithium aluminum hydride. Then the compound (IV) is enzymatically resolved according to the procedure described by Christian Triantaphylides et al., Tetrahedron Letters, 26(15), 1857 (1985) to give (1S)-1-methyl-2-norbornanol (V). This optically active alcohol (V) is then oxidized with pyridine dichromate complex to give (15)-1-methyl-2-norbornanone (VI). The compound (VI) is reacted with methyltriphenylphosphonium bromide in the presence of n-butyllithium to give (1S)-2-methylenenorbornane (VII), which is then treated with peracetic acid to give an epoxy compound. This epoxy compound is treated with a boron trifluoride etherate complex to give (1S,2S)-1-methyl-2-norbornylaldehyde (VIII), which is then reduced with lithium aluminum hydride to give (1S,2S)-1-methyl-2-hydroxymethylnorbornane (IX).

Then, this alcohol compound (IX) is treated with p-toluenesulfonyl chloride to give a tosylate (X). This tosylate (X) is reacted with sodium cyanide to give (1S,2S)-1-methyl-2-cyanomethylnorbornane (XI). This compound (XI) is reduced with diisobutylaluminum hydride (DIBAH) to give (1S,2S)-1-methylnorbornane-2-acetaldehyde (XII). This compound (XII) is subjected to aldol condensation with nitroethane in the presence of potassium fluoride to give 1-[(1S,2S)-1-methyl-2-norbornyl]-3-nitro-2-butanol (XIII). This compound (XIII) is then reduced with Raney nickel to give diastereomers of 1-[(1S,2S)-1-methyl-2-norbornyl]-3-amino-2-butanol (XIV). These diastereomers are separated with (+)- and (-)-tartaric acids to give a (2R,3R)-compound (XV).

The resulting 1-[(15,25)-1-methyl-2-norbornyl]-(2R,3R)-3-amino-2-butanol (XV) is reacted with an active ester obtained by condensing monobenzylcarbobenzoxy-L-aspartate with N-hydroxy-5- norbornene-2,3-dicarboxyimide and cyclohexylcarbodiimide to give (2R,3R)-3-N-(carbobenzoxy-L-aspartyl-β-benzyl ester)amino-1-[(15,25)-1-methyl-2-norbornyl]-2-butanol (XVI). The protecting group of the compound (XVI) is reductively removed with palladium/carbon. Thus, (2R,3R)-3-N-(L-aspartyl)amino-1-[(1S,2S)-1-methyl-2-norbornyl]-2-butanol (XVII), i.e. one of the compounds of the present invention, is obtained. Reaction scheme (B): Reduction KF catalyst Raney Ni (+)-tartaric acid Reduction H₂/Pd-black

On the other hand, (2R,3R)-3-N-(L-aspartyl)-amino-1-[(25,3R)-3-methyl-2-norbornyl]-2-butanol can be obtained by the reaction scheme (B) as shown above.

Namely, a readily available starting material (+ )-camphor-10-sulfonic acid (I') (manufactured by Aldrich Chemical Co., Inc.) is treated according to the procedure described by Wolfgang oppalzer et al., Helvetica Chemica Acta, 67, 1397 (1984) and Tetrahedron, 42, 4035 (1986). The starting material is first reacted with phosphorus pentachloride and ammonia to give (+ )-camphor-10-sulfonamide (II'). This compound (II') is then reacted with sodium methylate to give an intramolecular imine (III'), the unsaturated bond of which is then reduced with lithium aluminum hydride to give a sultam (IV'). The sultam (IV') is reacted with crotonyl chloride to give an acylated compound (V'). This compound (V') is subjected to a Diels-Alder reaction with cyclopentadiene to give an adduct (VI'), the camphane ring portion of which is then removed with lithium aluminum hydride to give (5S,6R)-6-methyl-5-hydroxymethyl-2-norbornene (VII'). This compound (VII') is then catalytically reduced with palladium/carbon to give (2S,3R)-2-hydroxymethyl-3-methylnorbornane (VIII'). This compound (VIII') is treated in the same manner as described in the reaction scheme (A) with respect to (1S,2S)-2-hydroxymethyl-1-methyl-norbornane (IX) to give (2R,3R)-3-N-(L-aspartyl)amino-1-[(2S,3R)-3-methyl-2-norbornyl]-2-butanol (XVI'), which is another compound of the present invention.

The compounds of the present invention, i.e., (2R,3R)-3-N-(L- aspartyl)amino-1-(1-methyl-2-norbornyl)-2-butanol and (2R,3R)-3-N- (L-aspartyl)amino-1-(3-methyl-2-norbornyl)-2-butanol, are colorless and odorless powders and are easily soluble in water. A diluted aqueous solution thereof has sweetness properties very similar to those of sucrose and gives almost no unpleasant taste such as bitter taste, bad taste and unpleasant aftertaste. Table 1 shows the results of evaluations of their sweetness properties. In particular, the features of the compounds of the present invention are that they have excellent stability in acidic aqueous solutions and good heat stability.

The compounds of the present invention can be widely used in various foods, beverages, toothpastes, cigarettes and cosmetics to which sweeteners are usually added, regardless of their forms. For example, they can be used for non-alcoholic beverages such as fruit juice, fruit drinks and lemonade, lactic acid drinks and carbonated drinks, optionally in powder form, alcoholic beverages such as rice wine (sake), synthetic rice wine, fruit liqueurs and sweetened rice wine for seasoning (mirin), cold sweets such as ice cream and sorbet, fruits in syrup, condiments such as miso paste, soy sauce, sauce, vinegar, dressing, mayonnaise and ketchup, cakes such as rice cakes, bread, cakes, biscuits and crackers, chocolate, chewing gum, jelly, sweet bean jelly, jam, marmalade, modified milk powder, various seaweeds or haddock cooked in sweetened soy sauce, canned foods, delicacies made from domestic animal meat, meat products such as bacon, ham and sausage, fish meat products such as cooked fish paste (of the species, known in Japan as "kamaboko" and "chikuwa"), seasonings, lipsticks, and medicines for oral administration.

The compounds of the present invention can be used either in the form of a powder as they are or in the form of a solution in a suitable solvent. The amount of these to be added can be appropriately selected depending on for example, the nature of the compound, the purpose, the subject, the method of addition and the type and amount of other sweetener(s) and spice(s) to be used together, if any.

To further explain the present invention, the following examples, reference examples and use examples are given.

Pressures in kg/cm² are converted to Pa x 10⁵ by multiplying by a factor of 0.98 and to mmHg in Pa by a factor of 133.

example 1 1-1) Synthesis of 2-methyl-norbornanol (II)

A Grignard reagent was prepared from 96 g (4.0 M) of magnesium turnings and 553.0 g (3.9 M) of methyl iodide in 600 mL of absolute diethyl ether under a nitrogen gas stream in a conventional manner. 225.0 g (2.05 M) of norbornanone (I) (manufactured by Aldrich Chemical Co., Inc.) was added dropwise thereto at room temperature, and the resulting mixture was stirred at room temperature overnight. The reaction mixture was poured into 800 mL of a saturated aqueous solution of ammonium chloride. The organic phase was collected and dried over anhydrous magnesium sulfate. After evaporation of the ether, the residue was distilled under reduced pressure to give 250.1 g (theoretical yield: 97%) of the title compound (II) as a colorless oily material. Boiling point: 75ºC/18 mmHg.

1-2) Synthesis of 1-methyl-2-norbornyl acetate (III)

1 ml of 75% sulphuric acid was added to 240.0 g (1.9 M) of the alcohol (II) obtained in 1-1) above and 500 ml of glacial acetic acid, and the resulting mixture was stirred at 60ºC for three hours. After cooling, the reaction mixture was diluted with 3 l of water and then neutralized with sodium carbonate.

Then, it was extracted three times with 300 ml portions of diethyl ether and dried over anhydrous magnesium sulfate. After evaporation of the ether, the residue was distilled under reduced pressure to give 297.2 g (theoretical yield: 93%) of the desired compound (III) as a colorless oily material. Boiling point: 82-83 ºC/17 mmHg.

1-3) Synthesis of 1-methyl-2-norbornanol (IV)

290.0 g (1.73 M) of the acetate (III) obtained in 1-2) above dissolved in 300 ml of absolute diethyl ether was added to 65.2 g (1.7 M) of lithium aluminum hydride suspended in 800 ml of absolute diethyl ether at room temperature under a nitrogen gas stream. Then, the resulting mixture was heated to reflux for six hours and stirred at room temperature overnight. Then, 500 ml of a 5% aqueous sulfuric acid solution was slowly added to the reaction mixture with stirring under ice-cooling. The ether layer was collected and dried over anhydrous magnesium sulfate. After evaporation of the ether, the residue was distilled under reduced pressure to give 209 g (theoretical yield: 96%) of the title compound (IV) as a colorless oily material. Boiling point: 83ºC/20 mmHg.

1-4) Synthesis of (1S)-1-methyl-2-norbornanol (V)

209 g (1.66 M) of the alcohol (IV) obtained in 1-3) above and 332 g (1.66 M) of lauric acid were dissolved in 660 ml of n-hexane and 66 g of Lipase MY (manufactured by Meito Sangyo KK) was added thereto. The resulting mixture was allowed to react at 35 to 40°C with stirring for three days. After After filtering off the enzyme and evaporating the n-hexane, the residue was distilled under reduced pressure to give 79.6 g (theoretical yield: 76%) of the title optically active alcohol (V). Boiling point: 82ºC/19 mmHg.

1-5) Synthesis of (1S)-1-methyl-2-norbornanone (VI)

79.6 g (0.63 M) of the optically active alcohol (V) obtained in 1-4) was dissolved in 300 ml of methylene chloride, and the resulting solution was added dropwise to a suspension of 304 g (0.81 M) of pyridine dichromate in 700 ml of methylene chloride at room temperature. Then, the resulting mixture was stirred for three days. The resulting reaction mixture was diluted with 1 L of diethyl ether and filtered through Celite. The filtrate was dried over anhydrous magnesium sulfate, and the solvent was evaporated. The residue was distilled under reduced pressure to give 50.4 g (theoretical yield: 64.5%) of the title compound (VI) as a colorless oily material. Boiling point: 70 °C/20 mmHg.

[α]20D = +32.0º (c=3.09, CHCl₃)

1-6) Synthesis of (1S)-2-methylenenorbornane (VII)

143 g (0.4 M) of methyltriphenylphosphonium bromide was added to a mixed solution of 260 ml of a 15% solution of n-butyllithium in hexane and 500 ml of diethyl ether at room temperature under anhydrous conditions over 30 minutes. Then, the mixture was stirred at room temperature for four hours and 50.0 g (0.4 M) of the ketone (VI) obtained in 1-5) above was added dropwise thereto. The resulting mixture was heated with stirring overnight. The reaction mixture was cooled to room temperature and the triphenylphosphine oxide thus formed was filtered. 350 ml of water was added to the filtrate and the organic Phase was collected and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was distilled under reduced pressure to give 31.8 g (theoretical yield: 65%) of the title compound (VII) as a colorless oily material. B.p.: 135-137ºC.

1-7) Synthesis of (1S,2S)-1-methyl-2-norbornyl-aldehyde (VIII)

70 g of 40% peracetic acid were added to 31.0 g (0.25 M) of the alkene (VII) obtained in 1-6) above, 48.6 g (0.46 M) of sodium carbonate and 100 ml of methylene chloride at 10°C. The resulting mixture was stirred at room temperature overnight and then poured into 500 ml of water. The organic phase was washed twice with a saturated aqueous solution of Mohr's salt and then with water and then dried over anhydrous magnesium sulfate. After evaporation of the solvent, 38.6 g of a yellow oily material was obtained. 450 ml of benzene and 13 ml of a boron trifluoride-etherate complex were added thereto and the resulting mixture was vigorously stirred for one minute. 450 ml of water was added to the reaction mixture, and the organic phase was collected, washed with water and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was distilled under reduced pressure to give 20.1 g (theoretical yield: 58%) of the title compound (VIII) as a colorless oily material. Boiling point: 75ºC/15 mmHg.

1-8) Synthesis of (1S,2S)-1-methyl-2-hydroxymethylnorbornane (IX)

20.1 g (0.15 M) of the aldehyde (VIII) obtained in 1-7) above were dissolved in 20 ml of absolute diethyl ether, and the resulting solution was added dropwise to a suspension of 1.9 g (50 mM) of lithium aluminum hydride in 100 ml of absolute diethyl ether under a nitrogen gas stream at room temperature. The resulting mixture was stirred for six hr and then cooled with ice. 100 ml of 5% sulfuric acid was slowly added dropwise. The ether layer was collected and dried over anhydrous magnesium sulfate. After evaporation of the ether, the residue was distilled under reduced pressure to give 19.6 g (theoretical yield: 96%) of the title compound (XI) as a colorless oily material. Boiling point: 65-68ºC/1 mmHg.

1-9) Synthesis of (1S,2S)-1-methyl-cyanomethylnorbornane (XI)

19.0 g (0.14 M) of the alcohol (IX) obtained in 1-8) above and 54.3 g (0.28 M) of p-toluenesulfonyl chloride were dissolved in 200 ml of dry pyridine under a nitrogen gas stream, and the resulting mixture was stirred at 10°C for 8 hours. The reaction mixture was poured into water and extracted with 150 ml of benzene. The extract was washed with 1 N hydrochloric acid and then with water and dried over anhydrous magnesium sulfate. After evaporation of the solvent, 39.9 g (theoretical yield: 100%) of a tosylate (X) was obtained as a colorless oily material.

Then, 39.9 g (0.14 M) of the tosylate (X) and 7.6 g (0.16 M) of sodium cyanide were dissolved in 850 mL of dimethyl sulfoxide and reacted at 90°C for five hours. The reaction mixture was poured into 400 mL of a saturated aqueous ammonium chloride solution and extracted twice with 150 mL portions of methylene chloride. The organic phase was washed with water seven times and dried over anhydrous magnesium sulfate. After evaporation of the solvent, 19.5 g (theoretical yield: 98%) of the above nitrile derivative (XI) was obtained as a colorless oily material.

1-10) Synthesis of (1S,2S)-1-methylnorbornane-2-acetaldehyde (XII)

19.5 g (0.13 M) of the nitrile (XI) obtained in 1-9) above were dissolved in 200 ml of absolute ether under a nitrogen gas stream and 160 ml of a 1 M solution of diisobutylaluminium hydride in hexane was added dropwise thereto at room temperature. Then, the resulting mixture was stirred for 1 hour, poured into ice/water and acidified with 1 N hydrochloric acid. The organic layer was collected and dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was distilled under reduced pressure to give 19.0 g (theoretical yield: 96%) of the title compound (XII) as a colorless oily material. Boiling point: 45-47ºC/0.5 mmHg.

1-11) Synthesis of 1-[(15,25)-1-methyl-2-norbornyl]-3-nitro-2- butanol (XIII)

38.0 g (0.25 M) of the acetaldehyde derivative (XII) obtained in 1-10) above was dissolved in 60 mL of isopropyl alcohol under a nitrogen gas stream, and 1.62 g (28 mM) of potassium fluoride and 26.7 g (0.35 M) of nitroethane were added thereto. The resulting mixture was stirred at room temperature overnight. The reaction mixture was poured into 350 mL of diethyl ether, and the organic phase was collected, washed with water and then with 0.5 N hydrochloric acid, and dried over anhydrous magnesium sulfate. After evaporation of the solvent, 55.9 g (theoretical yield: 97%) of the title compound (XIII) was obtained as a colorless oily material.

1-12) Synthesis of 1-[(1S,2S)-1-methyl-2-norbornyl]-3-amino-2-butanol (XIV)

10 g (44 mM) of the nitro alcohol derivative (XIII) obtained in 1-11) above was dissolved in 400 ml of ethanol and 8 g of a suspension of Raney nickel in ethanol was added thereto. The resulting mixture was sealed in a 1 liter autoclave under a hydrogen gas pressure of 25 kg/cm² and reacted therein at room temperature for eight hours. After completion of the reaction, the catalyst was filtered off. and the filtrate was concentrated under reduced pressure. The residue was distilled under reduced pressure to thereby obtain 7.2 g (theoretical yield: 83%) of the title compound (XIV) as a colorless oily material. Boiling point: 108 to 111ºC/1 mmHg.

1-13) Synthesis of 1-[(15,25)-1-methyl-2-norbornyl]-(2R,3R)-3- amino-2-butanol (XV)

45.0 g (0.23 M) of the amino alcohol derivative (XIV) obtained in 1-12) above was added to a solution of 34.1 g (0.23 M) of (+)-tartaric acid dissolved in 200 ml of ethanol, and the resulting mixture was reacted at 50 °C for one hour. The solvent was distilled off under reduced pressure to give 79.1 g of crude crystals (melting point: 160 to 162 °C). These crystals were recrystallized from hot ethanol four times to give 9.9 g of purified crystals. These crystals were thoroughly alkalized with an aqueous potassium carbonate solution and then extracted twice with 50 ml portions of methylene chloride. The extract was dried over anhydrous magnesium sulfate, and the solvent was evaporated. Thus, 5.89 g (15.7 mM) of a colorless oily material was obtained. To the thus-obtained product was added a solution of 2.36 g (15.7 mM) of (-)-tartaric acid dissolved in 50 ml of ethanol, and the mixture was reacted at 50°C for one hour. The reaction mixture was concentrated under reduced pressure, and the crude crystals thus formed were repeatedly recrystallized from hot ethanol four times. The purified crystals thus obtained were thoroughly alkalized with an aqueous potassium carbonate solution and treated in the same manner as described above. Thus, 0.15 g (0.17 mM) of the title compound (XV) was obtained as a colorless oily material. The characteristic values of these compounds are as follows:

[α]20D = +0.42º (c=0.5, methanol)

NMR (CDCl₃,δ):

1.08 (3H, s, ring-CH₃ group),

1.11 (3H, d, J=6.5 Hz,

2.77-2.84 (1H, m,

3.19-3.25 (1H, m,

1-14) Synthesis of (2R,3R)-3-N-(carbobenzoxy-L-aspartyl-β-benzyl ester)amino-1-[(15,25)-1-methyl-2-norbornyl]-2-butanol (XVI)

0.27 g (0.76 mM) of N-carbobenzoxy-L-aspartic acid β-benzyl ester (manufactured by Kokusan Kagaku KK) was dissolved in 10 ml of dioxane, and 0.14 g (0.76 mM) of N-hydroxy-5-norbornene-2,3-dioxyimide (manufactured by Peptide Kenkyusho KK) was added thereto. To the resulting mixture was added 0.17 g (0.83 mM) of dicyclohexylcarbodiimide under ice-cooling and stirring. The resulting mixture was stirred at room temperature for four hours. The dicyclohexylurea thus formed was filtered off, and 0.15 g (0.76 mM) of the amino alcohol (XV) obtained in 1-13) above dissolved in 3 ml of dioxane was added to the filtrate under cooling and stirring. Then, the reaction mixture was stirred at room temperature overnight to complete the reaction. After evaporation of the solvent, the residue was dissolved in 30 mL of ethyl acetate, washed successively with 10% citric acid, a 4% aqueous solution of sodium bicarbonate and a saturated sodium chloride solution, and then dried over anhydrous magnesium sulfate. After evaporation of the solvent, the residue was purified by silica gel column chromatography to obtain 0.22 g (theoretical Yield: 86%) to give the title compound (XVI) as a colorless oily material.

1-15) Synthesis of (2R,3R)-3-N-(L-aspartyl)amino-1-[(1S,2S)-1-methyl-2-norbornyl]-2-butanol (XVII)

0.22 g (0.41 mM) of (2R,3R)-3-N- (carbobenzoxy-L-aspartyl-β-benzyl ester)amino-1-[(1S,2S)-1-methyl-2-norborny1]-2-butanol (XVI) obtained in 1-14) above was dissolved in 10 ml of methanol and catalytically reduced in the presence of palladium black at room temperature for four hours. After the completion of the reaction, the catalyst was filtered off and the methanol was distilled off under reduced pressure. To the residue was added an appropriate amount of n-hexane to give 0.12 g (theoretical yield: 94%) of the title compound (w1I) as crystals. The characteristic values of these compounds are as follows:

Melting point: 147-150ºC

[α]20D = +0.68º (c=0.5, methanol)

Ms: 294 (M+ -18), 235, 161, 141, 125, 70 and 44 (base)

NMR (CD3OD,δ):

1.08 (3H, s, ring-CH₃ group),

1.14 (3H, d, J=6.7 Hz,

2.88 (2H, m, Asp-β-C 2 ,),

3.52 (1H, m,

3.84

and

4.18 (1H, m, Asp-α-C ).

The term "ASP" means aspartyl group.

Example 2 2-1) Synthesis of (2S,3R)-2-hydroxymethyl-3-methyl-norbornane (VIII')

40.5 g (0.29 M) of (5S,6R)-6-methyl-5-hydroxymethyl-2-norbornene (VIII') which was prepared from (+)-camphor-10-sulfonic acid (I') (manufactured by Aldrich Chemical Co., Inc.) was dissolved in 250 ml of methanol and catalytically reduced in a conventional manner, i.e., by stirring in the presence of palladium black at room temperature for 10 hours. After completion of the reaction, the catalyst was filtered off and the methanol was distilled off under reduced pressure. The residue was distilled under reduced pressure to give 39.5 g (theoretical yield: 95%) of the title compound (VIII') as a colorless oily material. Boiling point: 97 to 99°C/16 mmHg.

2-2) Synthesis of (2S,3R)-2-cyanomethyl-3-methyl-norbornane (X')

39.5 g (0.28 M) of the norbornane derivative (VIII') obtained in 2-1) above and 113 g (0.59 M) of p-toluenesulfonyl chloride were dissolved in 400 ml of dry pyridine under a nitrogen gas stream, and the resulting solution was stirred at 10 °C for 8 hours. After completion of the reaction, the reaction mixture was poured into water, extracted with 300 ml of benzene, washed with 1 N hydrochloric acid and then with water, and dried over anhydrous magnesium sulfate. After evaporation of the solvent, 82.3 g (theoretical yield: 100%) of a tosylate (IX') was obtained as a colorless oily material. Then, 82.3 g (0.28 M) of this tosylate (IX') and 15.7 g (0.32 M) of sodium cyanide were dissolved in 750 ml of dimethyl sulfoxide under a nitrogen gas stream and reacted at 90 °C for five hours. The reaction mixture was poured into 800 ml of a saturated aqueous ammonium chloride solution and extracted twice with 300 ml portions of methylene chloride. The organic phase was washed with water seven times and dried over anhydrous magnesium sulfate. After evaporation of the solvent, 40.2 g (theoretical yield: 96%) of the title nitrile derivative (X') was obtained as a colorless oily material.

2-3) Synthesis of (2S,3R)-3-methylnorbornane-2-acetaldehyde (XI')

40.2 g (0.27 M) of the nitrile derivative (X') obtained in 2-2) above was dissolved in 500 ml of absolute diethyl ether under a nitrogen gas stream, and 330 ml of a 1 M solution of diisobutylaluminum hydride in hexane was added dropwise at room temperature. After the addition was completed, the resulting mixture was stirred for 1 hour, then poured into ice/water and acidified with 1 N hydrochloric acid. The ether phase was collected and dried over anhydrous magnesium sulfate. After evaporation of the ether, the residue was distilled under reduced pressure to give 30.4 g (theoretical yield: 83%) of the above acetaldehyde derivative (XI'). Boiling point: 90 to 92 °C/15 mmHg.

2-4) Synthesis of 1-[(2S,3R)-3-methyl-2-norbornyl]-3-nitro-2- butanol (XII')

34.4 g (0.23 M) of the acetaldehyde derivative (XI') obtained in 2-3) above was dissolved in 50 ml of isopropyl alcohol under a nitrogen gas stream. 1.47 g (25 mM) of potassium fluoride and 24.2 g (0.32 M) of nitroethane were added thereto, and the resulting mixture was stirred at room temperature overnight. The reaction mixture was poured into 300 ml of diethyl ether and extracted with diethyl ether. The organic phase was collected, washed with water and then with 0.5 N hydrochloric acid. and dried over anhydrous magnesium sulfate. After evaporation of the ether, 50.6 g (theoretical yield: 97%) of the above nitro alcohol derivative (XII') were obtained as a colorless, oily material.

2-5) Synthesis of 1-[(2S,3R)-3-methyl-2-norbornyl]-3-amino-2-butanol (XIII')

10 g (44 mM) of the nitro alcohol derivative (XII') obtained in 2-4) above was dissolved in 400 ml of ethanol, and 8 g of a Raney nickel suspension in ethanol was added thereto. The mixture was sealed in a 1-liter autoclave under a hydrogen gas pressure of 25 kg/cm2 and reacted therein at room temperature for eight hours. After the completion of the reaction, the catalyst was filtered off and the filtrate was concentrated under reduced pressure. The residue was distilled to give 7.0 g (theoretical yield: 81%) of the above amino alcohol derivative (XIII') as a colorless oily material. Boiling point: 103ºC/1 mmHg.

2-6) Separation of (2R,3R)-1-[(2S,3R)-3-methyl-2-norbornyl]-3- amino-2-butanol (XIV')

33.0 g (0.17 M) of the amino alcohol derivative (XIII') obtained in 2-5) above was added to a solution prepared by dissolving 25.2 g (0.17 M) of (+)-tartaric acid in 150 ml of ethanol, and reacted therewith at 50 °C for one hour. The solvent was distilled off under reduced pressure to give 58.2 g of crude crystals (melting point: 154-156 °C). These crystals were repeatedly recrystallized from hot ethanol six times to give 3.16 g of purified crystals. These crystals were thoroughly alkalized with an aqueous potassium carbonate solution and extracted twice with 50 ml portions of methylene chloride. The extract was dried over anhydrous magnesium sulfate. After evaporation of the solvent, 1.64 g (8.3 mM) of the compound (XIV') were obtained as a colorless, oily material The characteristic values of this compound were as follows:

[α]20D = +19.3º (c=1, ethanol)

NMR (CDCl₃,δ):

0.95 (3H, s, ring C 3 group),

1.10 (2H, d, J=6.6 Hz,

2.71-2.78 (1H, m,

and

3.71-3.22 (1H, m,

2-7) Synthesis of (2R,3R)-3-N-(carbobenzoxy-L-aspartyl-β-benzyl ester)amino-1-[(2S,3R)-3-methyl-2-norbornyl]-2-butanol (XV')

2.97 g (8.3 mM) of carbobenzoxy-L-aspartic acid β-benzyl ester (manufactured by Kokusan Kagaku KK) was dissolved in 70 ml of dioxane, and 1.50 g (8.3 mM) of N-hydroxy-5-norbornene-2,3-dicarboxyimide (manufactured by Peptide Kenkyusho KK) was added thereto. The resulting mixture was stirred under ice-cooling, and 1.90 g (9.1 mM) of dicyclohexylcarbodiimide was further added thereto. The resulting mixture was stirred at room temperature for four hours. The dicyclohexylurea thus formed was filtered off, and 1.64 g (8.3 mM) of the amino alcohol derivative (XIV') obtained in 2-6) above dissolved in 10 ml of dioxane was added thereto under ice-cooling and stirring. The reaction mixture was stirred at room temperature overnight to complete the reaction. After evaporation of the solvent, the residue was dissolved in 200 mL of ethyl acetate and treated successively with a 10% aqueous citric acid solution, a 4% of the solvent, the residue was purified by silica gel column chromatography to give 2.33 g (theoretical yield: 83%) of the title compound (XV') as a colorless oily material.

2-8) Synthesis of (2R,3R)-3-N-(L-aspartyl)amino-1-[(2S,3R)-3- methyl-2-norbornyl]-2-butanol (XVI')

2.31 g (4.3 mM) of (2R,3R)-3-N- (carbobenzoxy-L-aspartyl-β-benzyl ester)amino-1-[(2S,3R)-3- methyl-2-norbornyl]-2-butanol (XV') obtained in 2-7) above was dissolved in 40 ml of methanol and catalytically reduced in the presence of palladium black at room temperature for four hours. After the completion of the reaction, the catalyst was filtered off and the methanol was distilled off under reduced pressure. To the residue was added an appropriate amount of n-hexane to give 1.21 g (theoretical yield: 90%) of the title compound (XVI') as crystals. The characteristic values of this compound are as follows:

Melting point: 127-133ºC

[α]20D = +23.8º (c=1.0, methanol)

Ms: 294 (M+ -18), 235, 162, 142, 125, 88, 70 and 44 (Base)

NMR (CD3OD,δ):

0.95 (3H, s, ring-CH₃ group),

1.16 (3H, d, J=6.9 Hz,

2.86 (2H, m, Asp-β-H),

3.53 (1H, m,

3.93 (1H, m,

and

4.16 (1H, m, Asp-α-C ).

Example 3

The (2R,3R)-3-N-(L-aspartyl)amino-1-[(2S,3R)-3-methyl-2-norbornyl]-2-butanol (XVI') obtained in Example 2, 2-8), which is hereinafter referred to as "Compound A", the (2R,3R)-3-N-(L-aspartyl)amino-1-[(1S,2S)-1-methyl-2-norbornyl]-2-butanol (XVII) obtained in Example 1, 1-15), APM and sucrose were each dissolved in water. The threshold value of each compound was determined by a limit method by a panel consisting of five experienced taste experts. Table 1 shows the results. Table 1 Sweetener Taste intensity threshold (%) Sweetening power (fold) Compound Sucrose

Example 4

Each of the compounds A and B, APM and L-aspartyl-DL-aminomalonic acid methylfenchyl ester as described in JP-A-49-30566, which will be abbreviated as "AMF" hereinafter, were dissolved in a 0.1 M phosphate buffer solution (pH 4.0) at a concentration of 0.2%. The resulting solution was kept at 80°C and the remaining Percent was monitored by high performance liquid chromatography over a period of six hours to thereby compare the stabilities of these solutions. The graph shown in the figure shows the results and clearly shows that the compounds of the present invention are superior to the control compounds in terms of stability.

Example 5

Compounds with the absolute configurations (a) and (b) of (RS,RS) and (S,S) were experimentally prepared and their sweetening power and sweetness properties were evaluated.

The sweetness properties were evaluated in the following manner.

A standard beverage was prepared by adding the following materials to 1600 ml of carbonated water:

Glucose 200g

Citric acid 1.5 g

Cola essence 2 ml

Water 400ml

Various sweeteners were added to the standard beverage, each in a specific concentration. The beverages thus obtained were stored at 37ºC for 26 days and then evaluated by 9 test experts according to the following scale. Table 2 shows the results.

Rating: 1 - extremely bad

2 - very bad

3 - bad

4 - partly bad

5 - good

6 - very good

7 - excellent

8 - very excellent

9 - extremely excellent Table 2 Compound Absolute configuration Melting point Sweetening power relative to sucrose Concentration Average value Aspartame

These results obviously show that the (S,S) compound had hardly any sweetness, while the (RS,RS) compound was inferior to the (R,R) compound not only in sweetening power but also in sweetness properties. From a commercial point of view, a sweetener compound should have a suitable sweetening power and excellent sweetness properties. Therefore, the compounds of the present invention meet these requirements.

Reference example 1-1: Separation of (2S,3S)-1-[(2S,3R)-3-methyl-2-norbornyl]-3-amino-2-butanol

58.2 g of the crude crystals obtained in the course of the procedure of Example 2 (2-6) were recrystallized twice from 95% hot ethanol. The mother liquor thus obtained was concentrated to give 5.1 g of the crystals. These crystals were repeatedly recrystallized from 95% hot ethanol three times. Thus, 0.85 g of a tartrate (melting point 200-201°C) was obtained as purified crystals.

Then, these crystals were thoroughly alkalized with an aqueous potassium carbonate solution and extracted twice with 50 ml portions of methylene chloride. The extract was dried over anhydrous magnesium sulfate. After evaporation of the solvent, 0.44 g (2.2 mM) of an oily material was obtained. The characteristic values of this compound are as follows:

NMR (CDCl₃,δ):

0.92 (3H, s, ring-CH₃ group),

1.11 (3H, d, J=6.5 Hz,

2.68-2.75 (1H, m,

and

3,11-3,16 (1H, m,

Reference example 1-2: Synthesis of (2S,3S)-3-N-(L-aspartyl)amino-1-[(2S,3R)-3-methyl-2- norbornyl ]-2-butanol

The (2S,3S)-1-[(2S,3R)-3-methyl- 2-norbornyl]-3-amino-2-butanol obtained in Reference Example 1-1 was treated in the same manner as Example 2, 2-7) to give (2S,3S)-3-N-(carbobenzoxy-L-aspartyl-β-benzyl ester)amino-1-[(2S,3R)-3-methyl-2-norbornyl]-2-butanol which was then treated in the same manner as in Example 2, 2-8) to give the title compound. This compound hardly showed any sweetness. The characteristic values of it are as follows:

Melting point 150-154 ºC

[α]20D = -30.0º (c=1, methanol)

Ms: 295, 235, 159, 143, 125, 70 and 44 (base)

NMR (CD₃OD,δ ):

0.92 (3H, s, ring-CH₃ group),

1.19 (3H, d, J=6.9 Hz,

2.95 (2H, m, Asp-β-CH₂),

3.48 (1H, m,

3.95 (1H, m,

and

4.22 (1H, m Asp-α-C ).

Reference example 2: Synthesis of 3-N-(L-aspartyl)-amino-1-d-α-fenchyl-2-butanol

214 g (1.4 M) of d-fenchone was reacted with 500 g of methyltriphenylphosphonium bromide to give 1,3,3-trimethyl-2-methylenebicyclo[2.2.1]heptane. This product was reacted with a synthesis gas to give fenchylacetaldehyde (b.p.: 75°C/1 mmHg), which was then subjected to aldol condensation with nitroethane. The resulting product was hydrogenated in the presence of a Raney nickel catalyst to give 1-d-α-fenchyl-3-amino-2-butanol (b.p.: 103°C/1 mmHg). This product was treated in the same manner as in Example 2, 2-7) to give 3-N- (carbobenzoxy-L-aspartyl-β-benzyl ester)amino-1-d-α-fenchyl-2-butanol, which was then treated in the same manner as in Example 2, 2-8) to give the title compound (melting point 129-136°C), which is hereinafter referred to as "Compound C".

For convenience, 1 g of each of Compounds A and B was thoroughly mixed with 99 g of glucose to give a powder sample with a concentration of 1%. Each amount described in the following application examples is expressed in terms of this 1% sample.

Usage example 1: cold dessert

Water was added to the following materials to make the total amount of 1000 g:

powdered starch syrup 180 g

Thickener 3 g

Citric acid 2 g

Table salt 0.1 g

Strawberry essence 1 ml

Compound A 8g

The resulting stock solution was frozen to produce a cold strawberry dessert that was refreshingly sweet and tasty.

Usage example 2: Fruit juice-free, carbonated drink

Water was added to the following materials to make the total amount of 400 mL:

Glucose 200g

Citric acid 1.5 g

Lemon essence 2 ml

Compound B 4.6 g

1600 ml of carbonated water was added to give a carbonated beverage which had excellent sweetness properties similar to those of sucrose.

Usage example 3: toothpaste

Water was added to the following materials to make the total amount of 1000 g:

Calcium hydrogen phosphate 500 g

Carboxymethylcellulose 11 g

Sodium lauryl sulfate 15 g

Glycerine 250 g

Compound A 0.4 g

Toothpaste flavor 9.5 g

Sodium benzoate 0.5 g

This composition was kneaded in a mixer to give a toothpaste that had a refreshing sweetness without any bitterness and gave preferable results.

Usage example 4: Sorbet

Water was added to the following materials to make the total amount of 1000 g:

powdered starch syrup 210 g

Stabilizer 3 g

Citric acid 1 g

Compound B 5.5 g

yellow dye no. 5 as desired

Orange flavour 1 g

The resulting composition was processed into a sorbet using a refrigeration machine. The product was comparable in taste to a regular one containing refined sugar.

The present invention provides 3-N-(L-aspartyl)-amino alcohol derivatives having stereospecificity as well as a sweetener containing them. These compounds have sweetening properties similar to those of sucrose and a high sweetening power and are stable in acidic aqueous solutions. Thus, they are very useful as a sweetener in various fields without any limitation.

Claims (4)

1. (2R,3R)-3-N-(L-aspartyl)amino-1-(1-methyl- 2-norbornyl)-2-butanol. 2. (2R,3R)-3-N-(L-aspartyl)amino-1-(3-methyl- 2-norbornyl)-2-butanol. 3. Sweetener containing (2R,3R)-3-N-(L-aspartyl)amino-1-(1-methyl-2-norbornyl)-2-butanol or (2R,3R)-3-N-(L-aspartyl)amino-1-(3-methyl-2-norbornyl)- 2-butanol. 4. A process for preparing a compound according to claim 1 or 2, according to the reaction scheme A or B disclosed here.